In recent years, formation of a full-color image by mixing of three-color toners of yellow, magenta, and cyan, or four-color toners of the above three colors, yellow, magenta, and cyan, and black using an electrophotographic system has been put to practical use.
In order to steadily record and hold record information, such as letters and images, an image receiving sheet used in the electrophotographic system generally comprises a substrate having thereon a receptive layer. This image receiving sheet is used, for example, for OHP (overhead projector) as information transmission means that is used in lecture meetings, schools, enterprises, other briefing sessions, exhibitions and the like.
In a printing mechanism utilizing the electrophotographic system, when an image formed on a photoreceptor is transferred onto an image receiving sheet, the surface electric resistivity of the image receiving sheet should be regulated because the toner is transferred by utilizing electric moment derived from electrification.
In this electrophotographic system, particularly in the case of printing on plastic base substrates, the resultant images considerably vary depending upon the electric resistivity of both sides of the substrates. In order to regulate the surface electric resistivity, antistatic agents have been used in the toner-receptive layer or the outermost surface layer.
For example, Japanese Patent laid-Open No. 81539/1973 discloses that, in order to improve the fixation of the toner and to provide good sheet carriability, a resin solution containing a surfactant such as typified by a quaternary ammonium base is coated on a sheet to specify the surface inherent resistance within a specific range.
In this method, however, since the surfactant has a low molecular weight, the concentration of the surfactant should be increased from the viewpoint of lowering the surface electric resistivity. This poses a problem that the coated face becomes sticky, or the surfactant migrates onto the contact face during the storage of the sheet. This changes the surface electric resistivity. Further, the mechanism used is such that the surface electric resistivity is regulated by ionic conduction. This creates a considerable difference in resistivity between the sheet under high humidity conditions and the sheet under low humidity conditions, making it difficult to always provide images having constant image quality.
Further, for example, Japanese Patent Laid-Open No. 238576/1987 discloses that an acrylic resin having a quaternary ammonium base is coated as a resin having antistatic properties to regulate the electric resistance. This acrylic resin can eliminate the migration of the surfactant onto the contact face. Since, however, the acrylic resin is an ion conductive material, the surface electric resistivity undergoes a change by two to three orders of magnitude upon a change in environmental conditions (temperature and humidity) around the sheet. This in turn causes a change in printability that makes it impossible to provide stable image quality.
Materials according to a system using the principle of electronic conduction is considered effective as materials having given resistance that is less likely to depend upon environmental conditions (temperature and humidity) around the sheet. For example, carbon black has a given resistance in electronic conduction due to mutual contact of the structure. Since, however, carbon black per se is a colored material, the addition of carbon black particularly on the image receiving face side results in coloration of the whole image receiving face that considerably limits the applications of the image receiving sheet.
Further, in current full-color copying machines or printers using the electrophotographic system, the formation of images on the conventional image receiving sheets, when a toner is melted and fixed at the time of toner fixation, is likely to cause offset of the toner and offset of a receptive layer in the OHP image receiving sheet provided with a receptive layer through the action of heat and pressure. This requires the adoption of a method wherein a silicone oil is coated by means of a fixation roll at the time of fixation of the toner.